The present invention relates to a dryer with at least one hopper for material to be dried and with at least one inlet line for dry air. The dry air is heatable with a heating device to a desired drying temperature and flows, after passing through the material to be dried within the at least one hopper, into a return line connected thereto.
In the hopper of such known dryers the material to be dried is stored. Such material is, in general, in the form of plastic granules or a flowable recycling material. Via the inlet line the dried air is introduced into the hopper in order to absorb the moisture of the material in the hopper. Via the return line the moist air is removed from the hopper. In these known dryers the air is circulated in an open circuit, i.e., the dry air is introduced from the exterior, heated to the drying temperature, and returned to the exterior, i.e., the surrounding atmosphere. This design results in a high energy consumption as well as heat loading of the surroundings.
In another dryer of the prior art the air is circulated in an open, mixed, or closed circuit. The return line is connected to the dry air inlet line. In the return line a valve is positioned for mixing the air with which valve the return line can be completely or partially closed relative to the dry air circuit. When this dryer of the prior art operates with an open circuit, i.e., the air mixing valve is closed, then the moist air is again returned into the surrounding atmosphere. When the air mixing valve is completely open or partially open, a portion of the moist air is returned into the dry air inlet line and is mixed with the dry air. This results in a reduced dew point since the moisture absorbing capacity of this mixed air in comparison to dry air is reduced.
It is therefore an object of the present invention to improve a dryer of the aforementioned kind such that it can be operated at reduced energy consumption, that only a small amount of heat is released to the surroundings of the dryer, that an optimal dew point is provided, and that the device is of a simple constructive design so that it can be manufactured inexpensively.